Hina is Hiei and Yukina's mother. She left the Glacial Village—home to her ice demon species—and had an affair with a male demon, resulting in twins. Yukina inherited her mother's ice apparition nature, but Hiei came out male and part fire demon, which made him a walking contradiction in their rigid society. The village elders saw him as a threat and ordered his execution.
Her closest friend Rui was tasked with the grim job of throwing the infant Hiei into the depths of Demon World. The anime suggests Hina took her own life shortly after, while the manga says she simply died in childbirth—either way, she didn't survive long after her son was taken from her.
Not much is known about Hina, but it's clear she was unconventional for an ice apparition. She was willing to break her people's laws to be with someone she loved, and she fiercely protected Hiei despite his dangerous hybrid nature. She pleaded for the elders to spare him, showing she loved him deeply even in those final moments.
Content compiled by AnimeList.moe from publicly available sources.
